Organs of the human body, such as the brain, kidney, and heart, are maintained at a constant temperature of approximately 37° C. Cooling is believed to be the most effective ischemia mitigator. More particularly, cooling of organs below 35° C. is believed to provide cellular protection from anoxic damage caused by a disruption of blood supply or by trauma. Also, cooling can reduce internal or external swelling associated with traumatic injuries.
Hypothermia is currently a useful medical tool and is sometimes performed to protect the brain or other organs from injury. Cooling of the brain is generally accomplished through whole body cooling to create a condition of total body hypothermia in the range of from 20° to 30° C. This cooling is accomplished by immersing a patient in ice, by using cooling blankets, or by cooling the blood flowing externally through a cardiopulmonary bypass machine. U.S. Pat. No. 3,425,419 (“Dato”) and U.S. Pat. No. 5,486,208 (“Ginsburg”) describe catheters for cooling the blood by circulating a cold fluid to create total body hypothermia. The systems of Dato and Ginsburg, however, are believed to be unsuitable for selective organ hypothermia because they do not provide for selective organ cooling. Hypothermia is achieved by circulating a cold fluid within each of the Dato and Ginsburg catheters, which are designed to be used in the great vessels like the inferior vena cava. Even if the catheters are placed in a selective vessel supplying an organ, there would be no manner of detecting when a desired temperature has been reached because there is no feedback system regarding the effect of the catheter on the selected organ temperature.
Use of total body hypothermia to provide organ protection is believed to have a number of drawbacks. First, it may create cardiovascular problems, such as cardiac arrhythmias, reduced cardiac output, and increased systemic vascular resistance, which side effects can result in organ damage. These side effects are believed to be caused reflexively in response to the reduction in core body temperature. Second, total body hypothermia is difficult to administer. Immersing a patient in ice water has its associated problems. Placement on cardiopulmonary bypass requires surgical intervention and specialists to operate the machine, and this procedure is associated with a number of complications, including bleeding and volume overload. And third, the time required to reduce the body temperature and the organ temperature is prolonged. Minimizing the time between injury and the onset of cooling is believed to produce better clinical outcomes.
Some physicians are believed to have immersed a patient's head in ice to provide brain cooling. Also, there are cooling helmets, or head gear, to perform a similar function. This approach suffers from the problems of slow cool down and poor temperature control due to the temperature gradient that must be established externally to internally. It is believed that complications associated with total body cooling, such as arrhythmia and decreased cardiac output, can be caused by cooling of the face and head only.
Selective organ hypothermia has been studied. See, for example, A. E. Schwartz et al., “Isolated Cerebral Hypothermia by Single Carotid Artery perfusion of Extracorporeally Cooled Blood in Baboons”, Neurosurgery, Vol. 39, No. 3, September 1996, pp. 577-582, and A. E. Schwartz et al., “Selective Cerebral Hypothermia by Means of Transfemoral Internal Carotid Artery Catheterization,” Radiology, Vol. 201, No. 2, November 1996, pp. 571-572. Utilizing baboons, blood is circulated and cooled externally from the body via the femoral artery and returned to the body through the carotid artery. These studies are believed to show that the brain could be selectively cooled to temperatures of 20° C. without reducing the temperature of the entire body. Subsequently, cardiovascular complications associated with total body hypothermia are not believed to have occurred.
Selective organ hypothermia is believed to have been attempted by perfusing an organ with a cold solution, such as saline or a perfluorocarbon. A type of selective organ hypothermia referred to as cardioplegia is performed to protect the heart during heart surgery. Cardioplegia has a number of drawbacks, including limited time of administration due to excessive volume accumulation, cost and inconvenience of maintaining the perfusate, lack of effectiveness due to temperature dilution from the blood, lack of a method to monitor hemodilation, and the decrease in the hematocrit of the blood supply to selected organs. Temperature dilution by the blood is a particular problem in high blood flow organs such as the brain. For cardioplegia, the blood flow to the heart is minimized; therefore, temperature dilution is minimized.
A conventional cooling catheter is believed to employ a feedback system to control the temperature of the cooled infusate. There is believed to be a need, however, for an intravascular system and method for corporeal cooling which provides for a more accurate and effective control of the volume and temperature of the cooled infusate and which is safer for the patient.